This invention relates generally to the field of ignition systems for gas turbine engines. More particularly, the present invention relates to a bipolar ignition system capable of delivering high-energy pulses to one or more gas turbine igniter plugs for reliable operation of the turbine in severe environments.
Gas turbine engines have found application in numerous areas of commerce and technology, from their use as jet aircraft engines to providing power for pumps and compressors in remote oil field or offshore locations. One characteristic of all such turbine engines is that, once started, the combustion occurring within the turbine is intended to be self-sustaining. That is, an ignition system for gas turbine engines is needed only for starting the engine. Once started, the combustion within the turbine is normally self-sustaining until the turbine is intentionally shut off by the operator or turns off spontaneously due to accidental variations of fuel or air supply or several other causes. [The igniter can also be operated continuously, as typically done for internal combustion engines, and occasionally necessary for gas turbine engines as well.] However, especially in airborne applications, it is very important that the engine be capable of reliable restarting under possibly severe conditions of temperature, pressure, humidity, fuel composition, etc. The circuitry for providing reliable, high-energy pulses for starting or restarting gas turbine engines is the subject of the present invention.
The basic operation of gas turbine igniter circuits typically involves the charging of a storage capacitor from a source of electrical power, followed by the sudden discharge of the capacitor through a spark-generating device ("igniter plug") inserted in the combustion region of the turbine. The sudden release of the energy stored in the storage capacitor through the igniter plug generates a spark for the ignition of the vaporized fuel adjacent thereto. In contrast to igniter circuits for driving spark plugs in typical Otto cycle internal combustion engines, turbine igniters are commonly required to deliver much higher energies per pulse through the igniter plug spark.
While simply described, the specific implementation for gas turbine igniters is subject to several technical challenges, due to the severe and variable environments in which the igniter system is required to operate, and the requirement of high energy delivery through the igniter plug for ideal turbine ignition systems.
One of the major technical challenges has involved the switch for discharging the energy in the storage capacitor rapidly through the igniter plug . This switch must be capable of rapidly turning on for discharge, but not suffer damage by the high currents and energies it must carry over relatively short periods of time. Currents carried by this discharge switch will typically exceed a thousand amps at peak values. This discharge switch typically must carry numerous repetitions of this pulse to insure reliable ignition of the gas turbine engine.
A common approach to the design of the discharge switch has centered around a gas discharge tube as the switching mechanism for rapid discharge of the storage capacitor (not to be confused with the spark generated by the igniter plug for ignition of the fuel within the turbine itself). This gas discharge switch commonly involves electrodes separated by a region of gas. When the critical discharge voltage across the electrodes is reached, a spark jumps the gap between the electrodes, leading to large current flow across the gap. The gas pressure and composition, the electrode geometry, spacing and material, all contribute in determining the voltage at which the gas discharge tube conducts and delivers the energy stored in the capacitor to the igniter plug. However, a serious drawback to the gas discharge tube has been its relatively short service lifetime, increasing maintenance costs for turbine operation. A more serious problem in many applications is the problem of a failed gas discharge tube and unreliable starting of the turbine. For these reasons, reliable solid state components have been finding wide usage in igniter circuits.
Most commonly, silicon controlled rectifiers ("SCRs") have been used to replace the gas discharge tube as the basic switching component for discharging the primary energy storage capacitor. In essence, an SCR is a semiconductor switch, capable of carrying current in one direction after it has been switched on by a "trigger" or "gate" pulse. Once switched to the conducting state, a typical SCR will remain conducting in its "forward" or conducting direction until switched off by interruption of current flow or forced reverse current flow. Typical SCRs will remain in the conducting state even in the absence of gating pulses although certain SCRs can be returned to the non-conducting state ("switched off") by negative gating pulses.
SCRs have proven to be much more reliable in actual operation than gas discharge tubes as a means for quickly discharging the energy in the capacitor through the igniter plug. As a solid state device, typical SCRs are much more tolerant of extreme conditions of temperature, humidity, etc. in which such igniter circuits are required to work.
However, use of SCRs in igniter circuits has brought additional challenges to the circuit designer. In general, SCRs must be protected from damage by excessive voltages in both forward and reverse directions: from excessively rapid changes in the voltages applied to the SCR and the currents passing therethrough, and from attempting to carry excessive currents through each SCR. All of this is to be accomplished while delivering maximum energy to the igniter plug. Various approaches to these problems have been taken.
A standard approach to SCR circuit technology is to use several SCRs in series to divide the applied voltages over several SCR's, thereby reducing the voltage any single SCR is required to endure. This has the drawback that failure of any one SCR in the series by means of an anode-cathode short circuit, will lead to overvoltages on all other SCR's in the series. Thus, failure of a single SCR by this mode will result in failure of all SCRs in the series and failure of the total device. Lozito el. al. (U.S. Pat. No. 5,053,913) have tackled this problem by requiring every SCR in the series to be capable of carrying the entire, undivided applied voltages. This redundancy certainly increases reliability in the event of an anode-cathode short occurring in the SCR. However, the increased costs of redundant SCR components in the circuit must also be considered, coupled with the increased voltage ratings (and costs) required of each separate SCR.
It is likewise common in the applications of SCRs to provide protection from rapidly changing voltages and currents by "snubber" circuits. Typically, a resistance-capacitance snubber circuit will be used in parallel with each SCR to provide a protective path around the SCR for rapidly changing voltages. In addition, inductance is typically provided in the SCR circuit to damp excessive changes in currents. Such techniques are well known "textbook" approaches to the use of SCR devices and also employed in the present invention.
However, the task for the designer of ignition exciter circuits is to provide maximum energy to the igniter plug with the most cost efficient, reliable circuit. Many designers have thus been led to consider "unipolar" ignition devices in which current flows through the igniter plug in one direction only. The advantage of such devices lies in part in that current is applied in only one direction to the SCR switch. Thus, the SCR switching circuits merely need to discharge the capacitor through the igniter plug, but need not withstand reverse voltages (for example, see the work for Frus, U.S. Pat. Nos. 5,065,073; 5,148,084; 5,245,252). However, (according to Frus) the resulting current delivered to the igniter plug is most effective when "shaped" in a variety of ways by means of a saturable inductors interposed between the SCR switch and the igniter plug.
The present invention has as its basic approach to the application of maximum energy to the igniter plug. The present invention typically applies 12 to 20 Joules of energy through the igniter plug. The present invention uses a bipolar circuit in which current flows through the igniter plug in both directions with peak values (typically) in excess of 2,000 amps. As a result, no waveshaping or conditioning circuitry is required between the SCR switch and the igniter plug. However, the use of bipolar ignition current leads to the application during half of the current cycle of significant reverse voltages to the SCR switch. The protection provided for the SCRs during this phase of the current flow cycle is a major feature of the present invention. In addition, the present invention includes (but is not limited to) a short-circuit protection mechanism to prevent discharge of the fully charged storage capacitor through a defective or shorted igniter plug.